1. Field of the Invention
The present invention relates to an ink jet recording apparatus which performs recording of data by using two or more types of ink, e.g., a penetrating-type ink used as a color ink and a drying-type ink used as black ink, as well as to an ink jet recording method using such an apparatus. More particularly, the present invention is concerned with a record data processing method which improves the quality of recorded images at and around the boundary between regions of different types of ink, and also an ink jet recording apparatus and method employing such a record data processing method.
2. Description of the Related Art
In recent years, office machines such as personal computers, wordprocessors and so forth are finding spreading use. These machines are usually equipped with print-out means for printing data which are output therefrom. Nowadays, various types of print-out or recording means have been developed and used, such as wire-dot type printers, thermal transfer-type printers, ink jet recorders, and so forth. Among these recording methods or systems, the ink jet recording system is being considered most promising, since it can provide color output of high quality at low cost, while coping with current demands for inexpensive color recording which is becoming strong as a result of spreading use of DTP (Desk Top Publishing) systems.
The nature of the recording ink is one of the factors that control the quality of images recorded by ink jet recording systems. In general, recording ink is an aqueous liquid and can broadly be classified into two types according to the mechanism or principle of fixing to recording mediums.
The ink of the first type is generally referred to as "penetrating-type ink" which, by virtue of an additive such as a surfactant added thereto, exhibits improved penetration into ordinary plain paper sheets such as copying paper sheets used as a recording medium. Thanks to the improved penetration, this type of ink exhibits an extremely short fixing time, e.g., several milliseconds to one second. The term "fixing time" is used to mean the time required for the ink to be fixed to a recording medium to such an extent that it is never transferred to any member which would be brought into contact with the recording medium. The ink of the second type is generally referred to as "drying-type ink" which does not significantly penetrate into the recording medium but is fixed by being dried on the surface of the recording medium. It takes about several tens of seconds for this type of ink to be fixed.
Inks of the penetrating type, by virtue of rapid penetration, are rather free from the problem of bleeding which tends to occur when dots of different inks of different colors are formed in close proximity of each other during recording of a color image. This type of ink, however, suffers from a problem in that coloring of dots cannot be enhanced due to the fact that the dye of the ink penetrates deep into the recording medium. Another problem encountered with this type of ink is that the recorded image tends to be made obscure due to low contrast at the boundary between printed and non-printed regions.
Meanwhile, ink of the drying type tends to suffer from the above-mentioned problem of bleeding at the boundary between dots of different color inks, although it exhibits superior coloring and contrast thanks to the tendency of slow penetration and minimal spread of the dye.
Thus, two types of inks have their own merits and demerits and are used selectively according to the nature of the recording to be performed.
In general, inks of the drying type are preferably used in printing of characters partly because use of different colors of inks in close proximity of each other takes place only seldom in this kind of printing and partly because the printing of characters requires a high level of contrast. Inks of the penetrating type are preferably used in printing of such images that require prevention of bleeding rather than keeping a high level of contrast, e.g., images of pictures of natural sceneries. Thus, Bk (black) inks which are commonly used as inks for printing characters are usually of the drying type, whereas color inks such as Y (yellow), M (magenta) and C (cyan) inks are usually penetrating-type inks.
It may become necessary, according to the type of the image to be recorded, that dots of color inks of the penetrating type are formed in close proximity of dots formed of the drying-type Bk ink. No substantial bleeding takes place at the boundaries between the regions formed by the penetrating-type inks of Y, M and C colors. The problem of bleeding, however, is critical at boundaries between a region formed by a penetrating-type color ink and a drying-type Bk ink.
FIGS. 21(a) and 21(b) are enlarged views of a portion of an image in which a region formed by a drying-type Bk ink and a region formed with a penetrating-type color ink are adjoining each other at a boundary indicated at A--A. More specifically, a half-tone region on the upper side of the boundary A--A is the color image region formed by the penetrating-type color ink, whereas the solid region below the boundary A--A is the Bk image region. It is preferred that the recording is effected without bleeding as shown in FIG. 21(a), however the image regions of the Bk ink and the color ink may be close to each other. Actually, however, there is a risk that the Bk ink spreads into the color image region formed by the color ink, thus causing bleeding at the boundary, as illustrated in FIG. 21(b).
Various measures have been proposed for the purpose of overcoming the above-described problem of bleeding, such as the method disclosed in European Patent Application Nos. 540173 and 590854. It has been proposed, in the specification of Japanese Laid-Open Patent Application No. 05-270582, a method having the steps of detecting a boundary between a region recorded by a penetrating-type ink and a drying-type ink, and substituting the image portion recorded by the drying-type Bk ink at the detected boundary with the penetrating-type ink. Use of a drying-type ink and a penetrating-type ink on adjacent dots or regions causes a color mixing at the boundary between these dots or regions, irrespective of whether which one of these two types of ink is recorded first.
More specifically, when two adjacent image regions are to be formed with Bk ink and a color ink, respectively, a conversion is performed so as to substitute the Bk color with process black (PCBk) constituted by yellow, magenta, cyan and black colors, in the boundary zone of the Bk image region having a predetermined width, e.g., a width corresponding to four dots, thereby preventing mixing of colors at the boundary between these two adjacent regions.
FIGS. 22(a) to 22(c) show the manner in which bleeding takes place at the boundary between two adjacent regions. In each of these Figures, the halftone region above the boundary indicated by a line A--A is the color image region, while the solid region below the boundary A--A is the Bk image region.
More specifically, FIG. 22(a) indicates an original image from which the boundary is detected by suitable means. A boundary zone defined between the boundary line A--A and a line B--B is the zone to be converted. The Bk image within the above-mentioned boundary zone is subjected to an image conversion into a color ink image to be formed by another color ink, whereby an image as shown in FIG. 22(c) is obtained.
It will be seen that, even the image after the conversion as shown in FIG. 22(c) contains bleeding of the Bk ink into the boundary zone between the lines A--A and B--B. However, since the image in the boundary zone between these lines is a Bk image zone which is constituted by a PCBk formed by adding color inks, the bleeding of the Bk ink into this zone does not produce any substantial appearance of "bleeding".
The quality of the image is improved significantly by the above-described control which is conducted by suitably using two types of ink, i.e., ink of the penetrating type and ink of the drying type.
It is to be understood, however, that the PCBk image formed by combined use of different colors through the above-described conversion is inferior to a Bk image formed by pure Bk ink, in terms of brightness and hue, from a strict point of view. Nevertheless, a better image can be obtained when the Bk dots are replaced by PCBk dots when the boundary zone between a color image region and a Bk region has a high duty ratio, because in such a case heavy bleeding is caused as explained in connection with FIGS. 21(a), 21(b) and FIGS. 22(a) to 22(c).
This method, however, suffers from the following problem. Namely, the PCBk conversion, i.e., conversion into PCBk dots, is effected unconditionally on the boundary zone defining the boundary between a Bk image region and a color image region, even when the bleeding problem is so slight as not to cause any critical problem. In such a case, the PCBk conversion undesirably impairs the quality of the recorded image as compared with the original image.
The PCBk conversion includes "four pixels boldfacing" in which a region formed by a single dot is expanded by an amount corresponding to four pixels in each of left, right, upward and downward directions from the single dot. This means that the region of the single dot is expanded into a square region constituted by 9 lines and 9 columns of pixels, i.e., 81 pixels. There is a substantial difference in the degree of necessity of PCBk conversion between the case where the duty ratio is so low that only one color dot exists in the above-mentioned region of 81 pixels and the case where the duty ratio is so high as to have all the 81 pixels colored. In the known method described above, the same boldfaced image is inconveniently obtained for both cases, because the same processing is applied to both cases, regardless of the significant difference in the degree of necessity of the PCBk conversion.
FIG. 23(a) to 23(d) illustrate an example of the above-described inconvenience. In FIG. 23(a), a belt-like solid area indicated by solid black is a Bk image region, while the area which is on the upper side of the Bk image region and which contains island-like or isolated dots is a color image region. FIG. 23(b) shows a state of the dots after the boldfacing in which each color dot is expanded to cover 81 pixels around the color dot, conducted for the purpose of detecting the boundary between two image regions. The area of the 81 pixels partly overlaps the original Bk region. The boundary zone is detected as the logical product (AND) of the boldfaced color image and the original Bk image region. Thus, the overlap area is determined as the boundary zone, as shown in FIG. 23(c).
All the pixels contained in this boundary zone are subjected to conversion. In this case, since the original Bk image region is a belt-like area of 3-dot width, all the pixels in the original Bk image region are converted into PCBk dots, as shown in FIG. 23(d). In the illustrated case, almost no bleeding is caused at the boundary, because of the small duty ratio in the color image region, as will be easily imagined from FIG. 23(a). Nevertheless, PCBk conversion is performed in the same way as that conducted for the case where the duty ratio in the color image region is 100%, so as to effect the substitution with PCBk even on the dots which actually do not require such a conversion.
This problem is due to the large area of the boldfacing which includes 81 pixels around each color dot and, therefore, would be overcome to some extent by reducing the area of the boldfacing down to 9 pixels, i.e., by two dots in each of left, right, upward and downward directions. Such a small amount of boldfacing, however, does not provide sufficient anti-bleeding effect, because the bleeding may take place beyond the boundary zone. The span of two pixels is as small as 0.14 mm in current major printers having resolution of 360 DPI (dot/inch). Such a small or narrow PCBk conversion zone can never provide a satisfactory anti-bleeding effect.
When the image to be recorded includes a Bk image region against a color image region of a low duty ratio, no substantial problem is caused by PCBk conversion, because the conversion converts only a portion of the contour region of the large Bk image region.
PCBk conversion, however, produces a noticeable detrimental effect when, for example, characters are to be printed with Bk ink against a halftone color background. It will be understood that no substantial bleeding takes place even when the recording is performed without the PCBk conversion. The characters printed with the Bk ink exhibit a high level of contrast which is peculiar to the Bk ink, thus offering high quality of the recorded image. The character image usually is constituted by line images having small widths. The following problem is therefore encountered when the recording is performed with PCBk conversion. Namely, almost all the pixels constituting the narrow line images are undesirably converted into PCBk dots, thus failing to provide the high level of contrast which is essential in the recording of this type of image. Thus, the size or area over which the PCBk conversion is effected should be minimized without reducing the anti-bleeding effect, so as not to impair the high level of contrast which is to be presented by the Bk ink.
This problem will be discussed in more detail with specific reference to FIG. 12 which shows a record image containing black characters A, B, C, D and E against a color gradation background. According to the above-described method, PCBk conversion is effected on the 4-dot-width boundary zones (shown as white) of all the black characters. More specifically, the PCBk conversion is effected equally and unconditionally on the 4-dot width boundary zones, regardless of variation in the printing duty ratio of the background. It is true that this PCBk conversion in one hand can prevent color mixing at the boundary, however the duty of the color background may be high but on the other hand poses a problem in that the PCBk zones at the boundary zone of the character are less distinctive in the lower part of the whole image where the duty ratio of print of the background is low. This is attributable to the fact that the contrast of black characters against the background as recognized by human eyes is higher in the area where the print duty ratio of the color background is lower.